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abstract sclass Either<A, B> {
  bool isA() { false; }
  bool isB() { false; }
  A a() { throw _failMe(); }
  B b() { throw _failMe(); }
  A aOpt() { null; }
  B bOpt() { null; }
  
  RuntimeException _failMe {
    fail("Either object is of wrong type: " + shortClassName(value));
  }
  
  toString {
    ret "Either" + (isA() ? "A" : "B") + "(" + value + ")";
  }
}

sclass EitherA<A, B> extends Either<A, B> {
  A value;
  
  bool isA() { true; }
  A a() { ret value; }
  A aOpt() { ret value; }
}

sclass EitherB<A, B> extends Either<A, B> {
  B value;

  bool isB() { true; }
  B b() { ret value; }
  B bOpt() { ret value; }
}